Cooperation in humans is a puzzle from the perspective of classical economic and evolutionary models. In recent years, laboratory experiments using economic games have revealed that humans show altruistic cooperation towards non-kin in anonymous one-shot encounters. What needs to be explained are the psychological, neural and evolutionary mechanisms that give rise to altruistic cooperation that is thought to be uniquely human. Two psychological mechanisms proposed to explain altruistic behaviour are sense of fairness and trust. Sense of fairness is usually investigated by using the Ultimatum game. Behavioural findings in the Ultimatum game show that humans reward behaviours that conform to fairness norms and that they punish behaviours that violate fairness norms, even it is not in their self-interest to do so. Activation of reward centres in the brain during such behaviour suggests that they get pleasure from doing so. Trust is investigated by using the Trust game. Behavioural findings in the Trust game show that humans make generous offers at first and they punish their partner if their trust is violated. At the neural level, it has been shown that administration of a neuropeptide that promotes social attachment increases trusting behaviour and that punishing violations of trust activates reward centres in the brain. These findings suggest that in economic exchange situations, humans take into account social, as well as economic, preferences, and that such behaviour is underpinned by special psychological mechanisms. Debate continues between proponents of individual selection and group selection as to how best to explain the evolutionary basis of altruistic cooperation.